In most computing systems, low level instruction code is used as an intermediary between the hardware components of the computing system and the operating software and other high level software executing on the computing system. In some computer systems, this low level instruction code is known as the computer Basic Input and Output System (“BIOS”). The BIOS provides a set of software routines that allow high level software to interact with the hardware components of the computing system using standard calls.
Because of limitations of the BIOS in many PC-compatible computers, a new specification for creating the firmware that is responsible for booting the computer and for intermediating the communication between the operating system and the hardware has been proposed. The new specification is called the Extensible Firmware Interface (“EFI”) specification and is available from INTEL CORPORATION.
The EFI specification describes an interface between the operating system and the system firmware. In particular, the EFI specification defines the interface that platform firmware must implement, and the interface that the operating system may use in booting. How the firmware implements the interface is left up to the manufacturer of the firmware. The EFI specification provides protocols for EFI drivers to communicate with each other, and the core provider functions such as allocation of memory, creating events, setting the clock, and many others. This is accomplished through a formal and complete abstract specification of the software-visible interface presented to the operating system by the platform and the firmware.
Both BIOS and EFI utilize the system management mode (“SMM”) provided in microprocessors available from INTEL CORPORATION and AMD CORPORATION. SMM is a special-purpose operating mode for handling system-wide functions like power management, system hardware control, or proprietary OEM-designed code. It is intended only for use by system firmware, not by applications software or general-purpose system software. The main benefit of SMM is that it offers a distinct and easily isolated processor environment that operates transparently to the operating system or executive and software applications.
When SMM is invoked through a system management interrupt (“SMI”), the central processing unit saves the current state of the processor (the processor's context), then switches to a separate operating environment contained in a special portion of random access memory (“RAM”) called the system management RAM (“SMRAM”). While in SMM, the microprocessor executes SMI handler code to perform operations such as powering down unused disk drives or monitors, executing proprietary code, or placing the entire computer in a suspended state. When the SMI handler has completed its operations, it executes a resume (“RSM”) instruction. This instruction causes the microprocessor to reload the saved context of the processor, switch back to protected or real mode, and resume executing the interrupted application or operating-system program or task.
The EFI is capable of providing an execution mode within the SMM and of providing services to other applications executing within the SMM. In particular, the EFI specification requires that a memory manager service be provided for use by program code executing within the SMM. It is, however, difficult to implement a memory manager program in the SMM due to the memory restrictions and other limitations imposed by the SMM. It is with respect to these considerations and others that the present invention has been made.